The advent of volume phase holography has stimulated research into better materials for recording and readout of the stored holographic information. Volume phase holograms are produced by changes in refractive index of selected portions of the recording medium, as contrasted with other holographic recording media, such as absorption holograms, which depend upon changes in optical density, or relief holograms which depend upon the formation of surface relief patterns for example, for holographic recording. Volume phase holography is more efficient than other systems since very little light is absorbed by the recording medium during readout, and they can be employed in thick sections, whereas other recording media can only be employed in sections of one millimeter or less in thickness.
Recording of volume phase holograms in said recording media consists of generating an interference pattern through the interaction between a coherent reference light beam and an object light beam. The object beam carries the spatial modulation corresponding to the image to be recorded. The light pattern formed by the interference of the object beam and the reference beam causes a change in the index of refraction of the recording media. The refractive index pattern produced thereby is a three-dimensional pattern which is representative of the object recorded. Readout is done with coherent light travelling in the direction of the reference beam and is phase modulated in accordance with the refractive index pattern, thereby reproducing the object beam through wavefront reconstruction.
Various materials which change their index of refraction upon exposure to coherent light have been investigated in the past with some success. For example, single crystals of electro-optic materials such as lithium niobate are known to provide volume phase holographic recording media. However, their efficiency is low and, in untreated crystals, the holograms are erased when coherent readout light of the same intensity as that used for recording is directed at the crystal. Doped crystals are also known and these provide higher efficiency holograms and can be treated so that the holographic information is made permanent. However, these crystals are difficult and expensive to obtain and the treatment requires additional equipment and processing steps.
Organic materials have been disclosed as holographic recording media also. Some change their index of refraction by changes in density brought about by photopolymerization. Others do so by photoreversible dimerization of guest molecules.